I love the Goto Anything menu, I use it religiously. I also use the more specific Goto menus, like the line, or symbol ones.

I find myself often hitting the keyboard command for another Goto action while using the Goto Anything menu. I think this addition would be useful and easy to implement.

Here's an example of how it might work:

Hit Command-P, start typing the name of the file and narrow it down, highlighting the one you're after.

Now press Command-R, you are now searching through symbols in the highlighted file. The string in the text field for the Goto Anything menu shows an "@" appended. You can backspace to return to the previous state.

Instead of using the @, # or : character you could always use the modifier key bindings. That would provide a single key binding regardless of context. For line number you would always know Command-G. I find that I am often looking at the preview of a file that the Goto Anything menu has open, so my brain is wired for file key bindings.

The @, # and : characters are only useful in the context of the menu– it seems more consistent and intuitive, to me, to just use the modifier based key bindings.